Exposed geomembrane covers: Part 2 - geomembrane restraint

Transcription

1 Expose geomembrane covers: Part 2 - geomembrane restraint By Gregory N. Richarson, Ph.D., P.E., principal of GN Richarson an Assoc. In the previos Designer s Form, esign proceres were presente to evalate the tension in expose geomembrane covers (EGCs) e to win-p lift loaing. his article explores alternative means of proviing anchorage to the EGC to resist sch plift forces. In general, this will involve attempting to se conventional featres of a final cover, (e.g. roaways, swales, etc.) to provie a portion of the reqire restraint. In aition to proviing mass to act as a restraint, sch featres still mst serve their primary fnctions. hese fnctions incle proviing access for roaways an srface-water control for swales an own chtes. hs, in many ways, the system of primary anchors se to resist win plift of the expose geomembrane will mirror featres of the complete final cover. (a) (b) (c) Figre : Examples of anchor trenches (a, b) an a bench (c). In this article, the varios anchorage systems are sbivie into three categories: () horizontally oriente anchors, (2) vertically oriente anchors, an (3) seconary anchors. Horizontally oriente anchors se final cap featres sch as swales an roaways that are primarily horizontally oriente. Vertically oriente anchors se featres sch as own-chtes that are primarily vertically oriente. Seconary anchors generally o not reflect a final featre of the complete cap, bt are interim points of anchorage. hs, the final anchorage system may incorporate several categories of plift anchorage. Horizontally oriente anchors Figre shows three types of horizontally oriente anchors propose by Giro (999). wo of these horizontally oriente anchors, b an c, moify sie-slope swales or roaways to create plift anchorage. he thir, a, is a moifie application of the stanar anchor trench. Anchor Bench Drainage swale moifications probably are the most intitive anchorage. hey provie several specific alternatives for the esigner that serve to illstrate the general esign approach. Figre 2 shows the general geometry of an anchor bench appropriate for a roaway or swale application. he EGC tensions,, are calclate sing the proceres iscsse in Part of this article. Using geometry, the EGC tensions can be resolve into forces vertical an horizontal to the horizon at the base of the anchor bench, as shown in Figre 3. he geomembrane tensions rece the sta- Reprinte with permission of Instrial Fabrics Association International. θ Direction R R α δ a Direction R Direction normal to the base of the bench Figre 2: Forces acting on an anchor bench when the geomembrane is plifte by win action. δ θ

2 bility of the bench anchor in two istinct ways: () the vertical components of the tension rece the effective weight of the bench an therefore the base friction, an (2) the horizontal component of the lower EGC acts to pll the anchor off the bench. he latter is of particlar concern if the tension in one of the EGCs is significantly larger than in the other. For this general case, Giro (999) shows how to etermine the minimm anchor weight reqire in Figre 4. In this eqation, min DON is the minimm weight if the EGC tensions rag the anchor own the slope, min UP is the minimm R 2 Figre 3: Decomposition of the forces weight if the EGC tensions rag the anchor p the slope, is the tension in the geomembrane below the anchor, is the tension in the acting on an anchor bench into vertical components (sbscript ) an components parallel to the base of the anchor geomembrane above the anchor, ß is the slope angle below the anchor, ß is the slope angle above the anchor, ß a is the slope angle immeiately below the anchor, θ is the bench (sbscript 2). angle between the plifte EGC an the slope below the anchor, θ is the angle between the plifte EGC an the slope above the anchor, an δ is the interface friction angle between the anchor an the EGC. he minimm weight base on vertical plift is given by H 2 R α R a 2 H Direction normal to the base of the bench minplift = sin (Θ ) + sin (Θ ) However, Giro showe that min DON > minplift an min UP > minplift. his means that the sliing conitions govern for a bench anchor an not the pre plift conition. A key point in the constrction of sch bench anchors can best be mae by looking at the two esign etails shown in Figres 5a-5b. he leaing ege of the bench anchor will be lifte by the tension in the EGC, an mst be stabilize in some manner to prevent raveling of the soil. Figre 5a shows the bench anchor etail se at the Delaware Soli aste Athority EGC project (Germain, et al, 996). min DON = ± cos (θ + δ + a )+ cos (θ + δ ) UP sin (δ + a ) Figre 4: Minimm anchor weight reqire. his esign encapslates the anchor soil in a geomembrane to contain it an prevent its loss. Alternative containment col se trf reinforcement mats (RMs) or geogri reinforcement. Figre 5b shows a bench roaway anchor etail that this athor evelope for caps at the Department of Energy s Oak Rige facility. Here, containment of the eges is provie by gabions or timber cribbing. Anchor rench An anchor trench provies passive earth pressre against the sie of the trench to resist the reslting horizontal component of the EGC tensions. Koerner (997) presents a goo iscssion Reprinte with permission of Instrial Fabrics Association International. 2

3 of the horizontal restraint of conventional anchor trenches. he net horizontal resistance of the anchor trench is given as P P -P A. P P is the passive resistance of the trench wall the anchor is being plle into an P A is the active force of the sie the anchor is being plle away from. he general eqations for P P an P A for the anchorage trenches are as follows: P A = / 2 γ 2 K a P P = / 2 γ 2 K p 2 aste Graing Soil 2.5 DSA Swale Anchor Protective Geomembrane Varies Final Graing Layer Daily/Intermeiate Layer Graing Soil Expose Geomembrane Figre 5a: Detail of swale bench anchor se at the Delaware Soli aste Athority EGC project. here γ is the nit weight of the soil forming the trench, is the epth of the trench, K a = tan2(45- φ/2), K p = tan2(45+ φ/2), an φ is the angle of shearing resistance of the soil forming the trench. Cshion Layer Gravel Srface Geocells Gabion Or imber Crbing hile P A an P P proce frictional forces on the sie of the trench that resist plift, these forces will be small in typical EGC applications. he resistance to plift is provie primarily by the weight of soil within the trench. his weight mst excee minplift previosly efine by Giro. Geotextile Gas Vent DOE Roaway Anchor Figre 5b: DOE roaway anchor. Geomembrane Care mst be taken in esign to ensre that the fll weight of soil within the trench will contribte effectively to plift restraint. Giro sggests that the sies of the trench mst be as vertical as possible, with a epth-to-with ratio of more than his ensres that the pressre acting on the geomembrane on the bottom of the trench is as niform as possible. An effective confinement of the soil in the trench mst be ensre by the esign. Vertical anchors Common vertical featres on lanfill sie slopes incle srface- water own-chtes an lanfill gas (LFG) collection pipes. A ternative EGCs have been constrcte to provie primary anchorage to the EGC geomembrane sing these vertical featres. his changes the orientation of the spans, e.g. horizontal vs. vertical, of the geomembranes an some esign concerns. Down-Chte Anchors Figre 6 shows a section throgh a rainage own-chte constrcte in the early 90s on the lateral perimeter of a mixe-waste lanfill EGC cap. he minimm weight of own-chte reqire to hol own plift forces from an EGC can be shown to eqal Reprinte with permission of Instrial Fabrics Association International. 3

4 min = left sin (Θ left ) + right sin (Θ right ) right an left simply enote the ECS tension on either size of the own-chte. Since ownchtes form a channel to contain flow, the nee for stabilization or reinforcement at the ege of the chte is not as ifficlt to achieve as for bench anchors. LFG Collection rench Anchors Figre 7 shows restraint of the EGC achieve by attaching it to LFG collection trenches that rn p the sie slopes an are evenly space horizontally. As with horizontal anchor trenches, the LFG collection trenches will not be move laterally by the horizontal forces from the EGC. Interim Geotextile Cshion Geomembrane Cap Revetment Mat eep Holes Gron Srface Revetment Mat Anchor Existing Stream Be Down Chte Anchor Figre 6: Section of a rainage own-chte. Vegetation Geotextile Anchor Geomembrane Anchor Compacte Backfill he minimm weight of the LFG collection trench can be calclate sing the previos soltion presente for own-chtes. Note that this neglects the friction forces on the sies of the trench. Seconary anchors he previos anchorage systems are base on featres of the final cover, that is, the EGC pls the lateral rainage, vegetative spport, an vegetative layer. hese anchorage systems lose their anchor role in the final cover, bt retain their fnamental role. Expose Geomembrane ' 0" el Geomembrane o HDPE Geogri Along Rib Gas Collection rench (30"X30") wo istinct types of seconary anchors incle both point an line anchors. he point anchors se commercially available earth anchors to provie restraint. o achieve a high loa capacity (>000 lbs.), these anchors are set in both the interim soil cover an the nerlying waste. Costing less than $00 each, they can be installe in the fiel to tensile capacities greater than 0,000 lbs. he ability to fielproof loa the tensile capacity of the varios point anchor systems is another benefit. aste LFG Collector Pipe Anchor HDPE Geogri Intermeiate Figre 7: Restraint of EGC by attachment to LFG collection trenches. Reprinte with permission of Instrial Fabrics Association International. 4

5 Remember that the primary markets for these point anchors are stabilize tility poles an win restraints for mobile homes. he installation simplicity of the anchors reflects this market. Seconary line anchors can be mae by filling polyethylene pipes with water or grot, then cap-stripping the pipe to the EGC. hese seconary line anchors provie spplemental plift protection, bt also provie a means of controlling srface-water flow over the EGC. his control of srface water can be critical ring the life of the EGC, since time of concentration of srface waters is minimal. hese seconary line anchors are particlarly helpfl if a vertically oriente primary anchorage system is se, since they o not break p the slope. he seconary line anchors increase anchorage an provie interim swales. Design factors of safety he economical anchorage of an EGC reqires the esigner to tilize featres of the final cap pls limite seconary anchorage systems. A minimm safety factor of.5 is recommene for bench anchors,.25 for trench anchors, an.0 for seconary anchors. All systems shol be inspecte if the esign win velocity is exceee at the site. his inspection, an proceres for sbseqent repairs, if reqire, shol be part of the facility operations manal. Smmary he athor acknowleges the benefits of a ecae of his own work on EGCs for the Department of Energy an the ongoing work by J.P. Giro to efine rigorosly the reqirements for sch systems. One interesting evice for proviing restraint to an EGC not covere by this article is a sction vent. Place in the pper portions of the slope, sction vents allow the win to raw air from beneath the EGC to proce a sction force below the EGC. Unfortnately, recent feeral reglations limit or ability to vent LFG irectly into the atmosphere. For non-ms lanfill applications, sch vents may be esirable. he thir an final installment of the EGC series will examine the criteria for selection of the type of geomembrane to se with seconary line anchors. References Chapman,., Peggs, I., Richarson, G., an ell, L Report of the ag-6 Interim Avisory Panel. DOE, Oak Rige, N. Germain, A., Gleason, M., an atson, R Design of an Expose Geomembrane Cap. SANA s 34th Annal International Soli aste Exposition, astecon. Giro, J.P., Gleason, M.H., an Zoinberg, J.G Design of Geomembrane Anchorage Against in Action. Geosynthetics International. Vol. 6, No. 6; Koerner, R.M Designing ith Geosynthetics. Forth EItion. Prentice Hall, Upper Sale River, NJ. Reprinte with permission of Instrial Fabrics Association International. 5